152 research outputs found
Detectability of Exoplanets in the Beta Pic Moving Group with the Gemini Planet Imager
We model the detectability of exoplanets around stars in the Beta Pic Moving
Group (BPMG) using the Gemini Planet Imager (GPI), a coronagraphic instrument
designed to detect companions by imaging. Members of the BPMG are considered
promising targets for exoplanet searches because of their youth (~12 MY) and
proximity (median distance ~35 pc). We wrote a modeling procedure to generate
hypothetical companions of given mass, age, eccentricity, and semi-major axis,
and place them around BPMG members that fall within the V-band range of the
GPI. We count as possible detections companions lying within the GPI's field of
view and H-band fluxes that have a host-companion flux ratio placing them
within its sensitivity. The fraction of companions that could be detected
depends on their brightness at 12 Myr, and hence formation mechanism, and on
their distribution of semi-major axes. We used brightness models for formation
by disk instability and core-accretion. We considered the two extreme cases of
the semi-major axis distribution - the log-normal distribution of the nearby F
and G type stars and a power-law distribution indicated by the exoplanets
detected by the radial velocity technique. We find that the GPI could detect
exoplanets of all the F and G spectral type stars in the BPMG sample with a
probability that depends on the brightness model and semi-major axis
distribution. At spectral type K to M1, exoplanet detectability depends on
brightness and hence distance of the host star. GPI will be able to detect the
companions of M stars later than M1 only if they are closer than 10 pc. Of the
four A stars in BPMG sample, only one has V-band brightness in the range of
GPI; the others are too bright.Comment: Accepted for publication in the Astronomical Journa
The atmospheric circulation of the super Earth GJ 1214b: Dependence on composition and metallicity
We present three-dimensional atmospheric circulation models of GJ 1214b, a
2.7 Earth-radius, 6.5 Earth-mass super Earth detected by the MEarth survey.
Here we explore the planet's circulation as a function of atmospheric
metallicity and atmospheric composition, modeling atmospheres with a low
mean-molecular weight (i.e., H2-dominated) and a high mean-molecular weight
(i.e. water- and CO2-dominated). We find that atmospheres with a low
mean-molecular weight have strong day-night temperature variations at pressures
above the infrared photosphere that lead to equatorial superrotation. For these
atmospheres, the enhancement of atmospheric opacities with increasing
metallicity lead to shallower atmospheric heating, larger day-night temperature
variations and hence stronger superrotation. In comparison, atmospheres with a
high mean-molecular weight have larger day-night and equator-to-pole
temperature variations than low mean-molecular weight atmospheres, but
differences in opacity structure and energy budget lead to differences in jet
structure. The circulation of a water-dominated atmosphere is dominated by
equatorial superrotation, while the circulation of a CO2-dominated atmosphere
is instead dominated by high-latitude jets. By comparing emergent flux spectra
and lightcurves for 50x solar and water-dominated compositions, we show that
observations in emission can break the degeneracy in determining the
atmospheric composition of GJ 1214b. The variation in opacity with wavelength
for the water-dominated atmosphere leads to large phase variations within water
bands and small phase variations outside of water bands. The 50x solar
atmosphere, however, yields small variations within water bands and large phase
variations at other characteristic wavelengths. These observations would be
much less sensitive to clouds, condensates, and hazes than transit
observations.Comment: 12 pages, 11 figures, 2 tables, accepted to Ap
High temperature condensate clouds in super-hot Jupiter atmospheres
Deciphering the role of clouds is central to our understanding of exoplanet
atmospheres, as they have a direct impact on the temperature and pressure
structure, and observational properties of the planet. Super-hot Jupiters
occupy a temperature regime similar to low mass M-dwarfs, where minimal cloud
condensation is expected. However, observations of exoplanets such as WASP-12b
(Teq ~ 2500 K) result in a transmission spectrum indicative of a cloudy
atmosphere. We re-examine the temperature and pressure space occupied by these
super-hot Jupiter atmospheres, to explore the role of the initial Al- and
Ti-bearing condensates as the main source of cloud material. Due to the high
temperatures a majority of the more common refractory material is not depleted
into deeper layers and would remain in the vapor phase. The lack of depletion
into deeper layers means that these materials with relatively low cloud masses
can become significant absorbers in the upper atmosphere. We provide
condensation curves for the initial Al- and Ti-bearing condensates that may be
used to provide quantitative estimates of the effect of metallicity on cloud
masses, as planets with metal-rich hosts potentially form more opaque clouds
because more mass is available for condensation. Increased metallicity also
pushes the point of condensation to hotter, deeper layers in the planetary
atmosphere further increasing the density of the cloud. We suggest that planets
around metal-rich hosts are more likely to have thick refractory clouds, and
discuss the implication on the observed spectra of WASP-12b.Comment: Accepted for publication in MNRAS, 10 pages, 1 table, 5 figure
Phase curves of WASP-33b and HD 149026b and a New Correlation Between Phase Curve Offset and Irradiation Temperature
We present new 3.6 and 4.5 Spitzer phase curves for the highly
irradiated hot Jupiter WASP-33b and the unusually dense Saturn-mass planet HD
149026b. As part of this analysis, we develop a new variant of pixel level
decorrelation that is effective at removing intrapixel sensitivity variations
for long observations (>10 hours) where the position of the star can vary by a
significant fraction of a pixel. Using this algorithm, we measure eclipse
depths, phase amplitudes, and phase offsets for both planets at 3.6 and
4.5 . We use a simple toy model to show that WASP-33b's phase offset,
albedo, and heat recirculation efficiency are largely similar to those of other
hot Jupiters despite its very high irradiation. On the other hand, our fits for
HD 149026b prefer a very high albedo and an unusually high recirculation
efficiency. We also compare our results to predictions from general circulation
models, and find that while neither are a good match to the data, the
discrepancies for HD 149026b are especially large. We speculate that this may
be related to its high bulk metallicity, which could lead to enhanced
atmospheric opacities and the formation of reflective cloud layers in localized
regions of the atmosphere. We then place these two planets in a broader context
by exploring relationships between the temperatures, albedos, heat transport
efficiencies, and phase offsets of all planets with published thermal phase
curves. We find a striking relationship between phase offset and irradiation
temperature--the former drops with increasing temperature until around 3400 K,
and rises thereafter. Although some aspects of this trend are mirrored in the
circulation models, there are notable differences that provide important clues
for future modeling efforts
Strong Near-infrared Spectral Variability of the Young Cloudy L Dwarf Companion VHS J1256–1257 b
Rotationally modulated variability of brown dwarfs and giant planets provides unique information about their surface brightness inhomogeneities, atmospheric circulation, cloud evolution, vertical atmospheric structure, and rotational angular momentum. We report results from Hubble Space Telescope/Wide Field Camera 3 near-infrared time-series spectroscopic observations of three companions with masses in or near the planetary regime: VHS J125601.92–125723.9 b, GSC 6214–210 B, and ROXs 42 B b. VHS J1256–1257 b exhibits strong total intensity and spectral variability with a brightness difference of 19.3% between 1.1 and 1.7 μm over 8.5 hr and even higher variability at the 24.7% level at 1.27 μm. The light curve of VHS J1256–1257 b continues to rise at the end of the observing sequence so these values represent lower limits on the full variability amplitude at this epoch. This observed variability rivals (and may surpass) the most variable brown dwarf currently known, 2MASS J21392676+0220226. The implied rotation period of VHS J1256–1257 b is ≈21–24 hr assuming sinusoidal modulations, which is unusually long for substellar objects. No significant variability is evident in the light curves of GSC 6214–210 B (<1.2%) and ROXs 42 B b (<15.6%). With a spectral type of L7, an especially red spectrum, and a young age, VHS J1256–1257 b reinforces emerging patterns between high variability amplitude, low surface gravity, and evolutionary phase near the L/T transition
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